Modulation systems and methods are widely used in transmitters to modulate an information input including voice and/or data onto a carrier. The carrier may be a final carrier or an intermediate carrier. The carrier frequency can be in UHF, VHF, RF, microwave or any other frequency band. Modulators are also referred to as "mixers" or "multipliers". For example, in a mobile radiotelephone, a modulator is used for the radiotelephone transmitter.
As is well known to those having skill in the art, modulation systems and methods for digital input signals generally include a Digital-to-Analog Converter (DAC) that converts the digital input signal into an analog signal. A low pass filter, also referred to as an "anti-aliasing filter", filters the analog signal to produce a filtered analog signal. A modulator modulates the filtered analog signal onto a carrier. The modulator includes a multiplier that is coupled to a local oscillator, such as a Voltage Controlled Oscillator (VCO), and to the filtered analog signal. The carrier including the filtered analog signal may then be transmitted by an antenna.
In modern communications systems, it is often desired to provide dual-mode modulation systems and methods that can modulate two types of communications signals. For example, in mobile radiotelephones, it is often important to provide a modulator that operates both in narrowband FM mode and in wideband Code Division Multiple Access (CDMA) mode. More particularly, in order to provide a mobile radiotelephone that can be used with both an IS-19 AMPS analog system and an IS-95 Direct Sequence Spread Spectrum (DSSS) wideband CDMA system, it is desirable to provide dual-mode modulation systems and methods.
In high performance communications systems, it also may be desirable to provide high carrier suppression. In order to provide high carrier suppression a low DC offset should be produced in the modulation system. For example, the required carrier suppression for FM modulation in an IS-19 AMPS analog system may be approximately -35 dBc. In order to provide an acceptable design margin, it may be preferred for the nominal carrier suppression to be -40 dBc, which can translate into a 14 mV differential DC offset signal when a 2V peak-to-peak differential information signal is generated in a balanced system.
Low DC offset in the digital input signal may be provided using conventional techniques. Unfortunately, however, the modulation system may generate its own DC offset. More specifically, the digital-to-analog converter and/or the low pass filter may generate DC offsets.
The DC offset that is generated in the digital-to-analog converter can be reduced using high performance digital-to-analog converters. Unfortunately, these digital-to-analog converters may be costly and complex. DC offset can be reduced in the low pass filter by providing a passive, off-chip filter with tight tolerance components. Unfortunately, such a passive off-chip filter may be costly and complex, and may consume excessive space in a portable radiotelephone.